1. Field of the Invention
This invention relates to chimney stacks or breechings of the type having an inner shell, or flue, through which flue gases flow, and an outer shell, for establishing an insulating space surrounding the flue. Insulated chimney stacks of this type, which are well-known to the prior art, are used to prevent cooling of the hot flue gases below their dew point before being emitted from the stack, since condensation of the flue gases on the interior surfaces of the flue can cause corrosion and it can as well undesirably affect the "drafting" properties of the chimney stack.
2. Description of the Prior Art
Prior art double-shelled chimney stacks in which the insulating space is sealed are subjected to high internal pressures when the trapped air, in the insulating space, is heated during smokestack operation. The design of these prior art chimney stacks must take into account such stresses, and these designs are necessarily stronger and hence more expensive and complicated than would otherwise be necessary. Furthermore, plant startup and shutdown cause repeated cyclic stresses to the shells and plates of the chimney stack, resulting in a tendency to failure by stress corrosion or metal fatique.
Also, because of the pressure differential which may exist between the interior of the flue, through which flow the potentially corrosive flue gases, and the insulating space, there may be leakage through small cracks or pinholes which may be present in the flue. During cooldown of the chimney stack, when the pressure in the insulating space drops below atmospheric pressure, corrosive flue gases may be drawn into the insulating space and condense on the interior surface of the outer shell of the chimney stack. Since the material of the outer shell is customarily selected for mechanical strength, and not for resistance to the corrosive flue gases, it may be readily attacked and damaged by such condensation. Condensation may also take place on the outer surface of the flue as well.
A technique known to the prior art involves the use of expansion joints, such as bellows or slip joints, between sections of the chimney stack or at stack openings, to permit expansion and contraction of the insulating space with temperature changes. Descriptions of these prior art approaches to these problems may be found in U.S. Pat. Nos. 3,363,591, 3,368,506, 3,487,795, 3,537,411, 3,669,042, and 3,727,566.
Techniques using bellows are not entirely successful because materials having suitable anticorrosion properties are not, in general, well suited to the fabrication of bellows. Furthermore, to be sufficiently flexible, the bellows must be quite thin and hence relatively weak and fragile. To obviate these problems, the bellows may be coated with a corrosion-resistant lining, but this approach is not wholly satisfactory because the lining tends to crack after numerous cycles of expansion and contraction.
While certain flexible lined materials, such as synthetics, fibers, etc., are available which can withstand the corrosive flue gases, these materials are not, for the most part, completely impermeable to the flue gases under conditions of pressure differential across the bellows.
The problems associated with double-shelled chimney stacks using slip joints to permit expansion and contraction of the insulating space include gas leakage through and around the slip joint seal, as well as seal deterioration caused by the corrosive flue gases.
In both the bellows and slip joint designs, pressure differences between the insulating space and atmospheric pressure are reduced but not entirely eliminated. As a result, there remains the possibility of leakage of flue gases through minute cracks or pinholes in the flue during cooldown of the chimney stack.